Having been available to protect content to be communicated via multicasting, via which the content is received by a plurality of communication devices, is a method for using a group key that is shared among the communication devices. Also known is a technology for causing a management device that manages the group key to distribute the group key to the communication devices. With this technology, the group key can be distributed at a lower communication cost, for example, when the group key is distributed from the management device using device keys, compared with when the group key is distributed to the individual communication devices.
To maintain the system security, it is preferable for the device keys to be updated on the regular basis. However, when there are many communication devices, it takes a long time for the device key of every communication device to be updated, for example, when an authenticated key exchange protocol established between two parties is used in distributing the updated device keys. With such a conventional technology, the availability of the system sometimes declines, because the group key cannot be distributed until updating of the device keys of the entire communication devices is completed.